It is known that the performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft powered chain drive or belt drive. It is also known that engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft. Heretofore, such changes in engine valve timing have been accomplished by a separate hydraulic motor operated by engine lubricating oil. However, this actuating arrangement consumes significant additional energy and it increases the required size of the engine lubricating pump because of the required rapid response time for proper operation of the camshaft phasing actuator. Further, these arrangements are typically limited to a total of 20.degree. of phase adjustment between crankshaft position and camshaft position, and typically such arrangements are two-position arrangements, that is, on, or fully phase adjusted as one position, and off, or no phase adjustment, as a second position.
The present invention is designed to overcome the aforesaid problems associated with prior art variable camshaft timing arrangements by providing a self-actuating, variable camshaft timing arrangement which does not require external energy for the operation thereof, which does not add to the required size of the engine lubricating pump to meet transient hydraulic operation requirements of such variable camshaft timing arrangement, which provides for continuously variable camshaft to crankshaft phase relationship within its operating limits, and which provides substantially more than 20.degree. of phase adjustment between the crankshaft position and the camshaft position. Prior U.S. Pat. Nos. which describe various systems of the foregoing type are 5,046,460 and 5,002,023, the disclosures of each of which are incorporated by reference. Those disclosures provide a pulse-width modulated (PWM) solenoid for varying the position of a control valve which actuates the VCT phase adjustment mechanism. Closed-loop control of those VCT systems was disclosed in U.S. patent application Ser. No. 07/847,577, the disclosure of which is incorporated herein by reference. This technology is acceptable for chain driven engines, but it is less than desirable in belt-driven engines where the PWM equipment may leak oil on the belt thus causing slippage and a decrease in engine efficiency.